Appendix A, which is a part of the present disclosure, is a microfiche appendix consisting of 1 sheets of microfiche having 17 frames. Microfiche appendix A includes a software program operable in a computer of a goniometric radiometer as described below.
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.
This and other embodiments are further described below.
1. Field of the Invention
The present invention relates to testing or characterization of light sources and, in particular, to the scanning and acquisition of far-field data from light sources, including optical fibers.
2. Discussion of Related Art
Conventional techniques for measuring or characterizing the radiation pattern of optical sources utilize a goniometer in combination with an optical detector. These xe2x80x9cgonioradiometricxe2x80x9d measurements are typically made by rotating the detector on a radial arm of the goniometer about the optical source (light source) to be measured. The detector scans through angles and measures light output as a function of angle. In accordance with another conventional technique, a light source is mounted to a rotating goniometer which scans the optical beam from the light source across a detector that is fixed in position. The mechanical scanning apparatus associated with the above-described known techniques often occupy a rather large volume due to the necessity of scanning at specified radii which are on the order of tens of centimeters or larger in some cases. In addition, the mechanical scanning apparatus typically does not allow for rapid positioning. This results in lengthy scan times, often as much as thirty minutes or more per individual scan. This translates to characterization times on the order of hours or days for a fill incremental scan of the radiation pattern of the source.
The fiber optic community needs a method for both fast and accurate measurement of optical fiber parameters. With advances in dense wavelength division multiplexing (DWDM) technology, the role of optical fibers is ever more demanding. With the increased deployment of standard fibers and the development of specialty fibers on the rise, the need to accurately characterize these fibers in greater numbers is paramount.
Two parameters of significant importance for predicting fiber optic system performance are the Mode-Field Diameter (MFD) and the Effective Area (Aeff). The MFD is used to evaluate losses due to mismatch at connections of fibers and fiber components, while Aeff is used to assess nonlinear effects. Determining these parameters from far-field radiation profile data has been specified as the reference method by the Telecommunication Industry Association/Electronic Industries Association (TIA/EIA). The accurate measurement of fiber parameters such as the Mode-Field Diameter (MFD) and Effective Area (Aeff) of single-mode optical fiber by the Direct Far-Field Method requires a system with dynamic range of at least 50 dB if not greater.
Requirements for measuring the Bidirectional Reflectance Distribution Function (BRDF) and the Bidirectional Transmittance Distribution Function (BTDF) of a scatterometer are even more demanding, with from approximately 90 to 120 dB of dynamic range needed.
To accomplish the dynamic ranges necessary for accurate measurements of fiber parameters or scatterometer data (BRDF or BTDF), conventional measurements utilize a single far-field scan with lock-in amplifier techniques and amplifier gain switching during the scan. These methods provide wide dynamic ranges, but typically results are obtained in times on the order of 30 minutes or more.
Therefore, there is a need for an apparatus and method for quickly measuring the far-field radiation profiles with high dynamic ranges from scatterometer sources, optical fibers, or other light sources.
In accordance with the present invention, an apparatus and method is presented for measuring the far-field radiation pattern around a light source. In some embodiments, the light source may be an optical fiber, in which case optical fiber parameters may be calculated from the measurement of the far-field radiation pattern. In some embodiments, the far-field radiation pattern is from a scatterometer source. In general, the optical source can be any source of radiation.
Some embodiments of the apparatus include a collector, a detector coupled to a detector electronics having a programmable gain, a motion controller for controlling the scanning of the collector, and a data acquisition computer. Some embodiments further include a radiation deflector (usually in the form of an entrance aperture mirror). The entrance aperture mirror deflects light from the light source to an entrance end of the collector. The collector is mounted to a cylindrical hub that is caused to rotate about the optical axis of the light source in a fashion controlled by the motion controller. In some embodiments, the light source can be rotated in order that a more three-dimensional scan of the far-field radiation profile can be obtained.
The collector is arranged to receive a scan of the radiation profile from the optical source as the entrance to the collector is rotated on the cylindrical hub in a plane around the optical source. In some embodiments, the optical source is fixed relative to the collector so that the collector scans through a plane intersecting with the radiation profile of the light source. In some embodiments, the light source can be rotated so that multiple scans of data corresponding to scans through multiple planes intersecting with the radiation profile of the light source is obtained. A three-dimensional radiation profile can be measured by obtaining scans through multiple planes intersecting with the radiation profile from the light source. In some embodiments, the light source is rotated with a positioning motor. In some embodiments, the entrance aperture mirror is rotated relative to the source so that individual intersecting planes of data are reflected into the plane defined by the rotation of the entrance to the collector. The entrance aperture mirror effectively selects a slice through the radiation pattern of the optical source being measured and the rotating collector scans through the light reflected by the entrance aperture mirror at that azimuthal slice and delivers it to the detector.
In embodiments with an entrance aperture mirror, the entrance aperture mirror can be rotatable about the optical axis of the optical source being measured. A step wise rotation of the entrance aperture mirror through 180xc2x0 is effective to characterize the entire three-dimensional radiation profile of the optical source. In some embodiments of the invention, the entrance aperture mirror is mechanically coupled to a stepper or servo motor and is controllably positioned by a motor controller, which is controlled by the data acquisition computer.
In some embodiments of the invention, the optical source is fixed relative to the plane traversed by the entrance aperture of the collector as it is rotated on the hub. In these embodiments, the apparatus collects data through one plane of the radiation profile from the light source. In some embodiments of the invention, the optical source can be rotated about a direction in the data collection plane defined by the rotation of the entrance to the collector. In that case, data taken from different rotational positions of the optical source represents data taken in different planes through the radiation profile of the light source.
In some embodiments of the present invention, the collector comprises an optical light fiber or a bundle of fibers. In another embodiment of the invention, the collector comprises a train of reflectors mounted in diametrically opposed fashion inside the rotating cylinder. The reflectors fold the optical path of the reflected light and thus increase the effective radius of measurement so that large radius scans can be obtained in a measuring instrument of compact geometry with dimensions on the order of tenths of meters.
In some embodiments, the collector has an exit end optically arranged in co-alignment with the axis of rotation of the rotating hub and separated from the detector by a small air gap. In some embodiments, the detector is included in the collector and rotates on the hub with the collector. The detector receives light from the collector and outputs a signal to the detector electronics. The detector electronics includes amplifiers and has an overall gain which is programmable by the data acquisition computer. The detector electronics is coupled to the data acquisition computer. In order to perform measurements over the entire dynamic range, the data acquisition computer accumulates and combines data from various settings of the gain of the detector electronics. The acquisition and combination of data from separate scans at each particular position of the light source relative to the collector from different gain settings allows acquisition of far-field profiles with dynamic ranges greater than could be obtained with any individual setting.
In some embodiments of the invention, the data-acquisition computer collects a set of averaged far-field radiation profile scans. Each scan in the set of averaged scans is taken with a corresponding gain setting of the detector electronics. An average scan for a particular gain setting is the average of a pre-selected number of individual scans at that gain setting. A set of averaged scans includes averaged scans for a number of selected gain settings of the device electronics.
A compiled far-field radiation profile in a particular plane of the radiation profile can be constructed from the set of averaged scans. In some embodiments, data from the scan corresponding to the lowest gain setting which has a value above a threshold value is utilized as the data for the compiled far-field radiation profile in the range of xcex81xe2x88x92 to xcex81+. Data from scans corresponding to higher gain settings which have a value above a threshold is then utilized to fill in the data for the compiled far-field radiation profile. For example, the data above the threshold value from the next lowest gain setting is utilized as data in the ranges xcex82xe2x88x92 to xcex81xe2x88x92 and xcex81+ to xcex82+ of the compiled far-field radiation profile and the scan from the next higher gain setting provides data for the range xcex83xe2x88x92 to xcex82xe2x88x92 and xcex82+ to xcex83+. The data acquisition computer, then, constructs the compiled far-field radiation profile from regions of the averaged scans of the set of averaged scans having data values above a threshold value.
A three dimensional scan of the radiation profile from the light source can be constructed by obtaining a compiled far-field profile in several planes through the radiation profile. The compiled far-field profile in each plane is constructed from multiple averaging scans in that plane with differing gain settings.
In some embodiments, the compiled far-field radiation profile is utilized to determine Mode Field Diameter (MFD), Effective Area (Aeff) and numerical aperture (NA) of optical fibers. Other fiber parameters may also be calculated based on the compiled far-field radiation profile from an optical fiber light source.
In some embodiments, the light source is the reflection or transmission of light through a material under test. In measurements of Bidirectional Scatter Distribution Functions (BSDF), the radiation profile of light reflected from the surface of a material or transmitted through the material from a laser beam incident on the material provides information regarding the material. In a Bidirectional Reflectance Distribution Function (BRDF) the reflectance radiation profile from the surface of a material around an incident laser beam is measured. In a Bidirectional Transmittance Distribution Function (BTDF) the transmission radiation profile through the material around an incident laser beam is measured. The radiation profile of light transmitted through the material from a laser beam incident on the material also provides information regarding the material. In some embodiments of the invention, both BTDF and BRDF data can be measured in a single scan.
These and other embodiments according to the present invention are further described below with reference to the following figures.